Taxiing around an airport can be a very daunting task, especially at night in a small aircraft. When maneuvering around an airport, it is important to be aware of all taxiway markings that exist and have good situational awareness of your surroundings. Failure to adhere to taxi instructions can place an aircraft in an undesired spot, like an ILS critical area, which may interfere with airport operations. This information is taught to our pilots enrolled in our fully online commercial pilot ground school course.

ILS Critical Area Definition

In TP-1490, Transport Canada defines critical areas as those where the presence of a vehicle or taxiing aircraft may possibly affect ILS signals. Depicted areas are theoretical and usually vary with individual sites. Actual critical areas can be defined only by experimentation and experience. When a suspected area may likely be a source of interference, that portion must be included as a part of the critical area. Additional hold short signs will be placed on taxiways and must be observed by aircraft and vehicles when instructed to by air traffic control.

ILS Critical Area Hold Short Sign — CAT I, CAT II, or CAT II holding position signs are installed to protect the ILS critical areas during ILS precision approaches. The inscription will consist of the designator of the runway and the inscription, as appropriate.

Protection of the ILS Critical Area

Air traffic services will protect the ILS critical area and glidepath signal when:

The ceiling is less than 1,000 feet or visibility is less than 3 miles, or both; and

The arriving aircraft is inside the FAF on an ILS approach

This is because when an aircraft is navigating on the ILS, fluctuations may occur when other aircraft or vehicles are moving through this critical area, causing interference with the signal. In some cases, the aircraft automation may follow momentary fluctuations, causing the aircraft to pitch or roll.

ILScritical — Typical information signs pilots may see if an ILS critical area is not at the same holding point of a runway. In this case, the aircraft is on taxiway B or Bravo.

Pilots Response

Pilots must be aware of ILS signal interference threats as well as flight display indications and autopilot functionality during manual or fully coupled ILS approaches.

There are times when pilots may be required to conduct autolands or practice low-visibility procedures. In these cases, the pilot must advise the controller of these intentions early enough so that the controller can either protect the critical area or advise the pilot that protection is not possible.

Embraer PDF with focus on glidepath — The red box shows the glidepath display on this Embraer primary flight display. This may become slightly erratic if an aircraft or vehicle enter the ILS critical areas near a runway.

Importance of the ILS Critical Area

Protecting the ILS critical area is mainly important during low visibility operations where there may be heavy fog or any other type of low visibility impacting the ability of aircraft to safely take-off and land. During normal routine operations where the visibility is good, cloud heights are good, and aircraft can comfortably see the landing runway, protecting the ILS critical area is not nearly as important. Again, unless the arriving aircraft is practicing an autoland or a low visibility procedure.

Examples

ILScriticalareaYYZ — This expired sample chart at Toronto Pearson shows that normally, aircraft would hold short of runway 05 right at the edge (where the three dots are beside the runway). During CAT II / III (very low visibility) operations, aircraft will hold parallel to the runway instead of abeam it.

YVRCritical — This is a sample chart from Vancouver which is expired. It shows that the ILS critical area for CAT II and CAT III (very low visibility) operations is well away from the runway area on taxiway A or Alpha.

Information and images were adapted from the Transport Canada Aeronautical Information Manual. Some information may be copyrighted. © His Majesty the King in Right of Canada, as represented by the Minister of Transport, (2023). This information has been reproduced with the permission of Transport Canada.

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